The invention relates to methods and apparatus for stabilizing the output wavelength of a laser, and more particularly, to a method and apparatus in which Zeeman splitting of atomic energy levels provides a signed error signal to stabilize laser light relative to a resonant optical transition.
Many laser applications require that the output wavelength of the laser be precisely controlled. In particular, it is sometimes required that the laser wavelength be locked relative to the wavelength of an atomic transition. One such application is the use of blue-green light pulses to communicate through ocean water.
Water strongly attenuates most wavelengths of electromagnetic radiation. However, a region of transparency exists for blue-green light. An exceptionally effective filter for light within the transparency region is provided by the cesium (Cs) resonance filter. The Cs resonance filter works by absorbing light at one wavelength in the blue and producing fluorescence at a longer wavelength in the near infrared. By using very brief pulses of light which are precisely tuned to the narrow absorption of the Cs resonance filter, it is possible for undersea receivers to receive communication signals and filter out competing light which causes noise. To use such a communication scheme, it is necessary to produce intense light pulses at a wavelength which precisely matches the Cs resonance filter. Very brief, intense pulses are required in order to improve the discrimination of the signal from environmental light sources which are slowly varying.
One method for locking a laser to the wavelength of the Cs resonance filter is to use Cs fluorescence to produce an electronic reference signal or error signal. The laser light wavelength is varied, and some of the laser light is used to excite Cs fluorescence. The fluorescence light is detected to produce a reference signal which is maximized when the laser light is at the desired wavelength. The disadvantage of this method is that the reference signal is not indicative of whether the laser wavelength is greater or less than the absorption wavelength. When the laser wavelength drifts it is necessary to adjust it back and forth to find the reference signal maximum. This process interrupts the communication signal for an unacceptable interval.
A second method for locking a laser wavelength to an atomic transition has been disclosed by R. D. Mead in U.S. patent application Ser. No. 4,912,716. Mead teaches a method and apparatus for stabilizing a laser frequency relative to Cs absorption which provides an error signal that indicates both the direction and magnitude of wavelength error. The method employs microwave frequency modulation of the laser wavelength to produce sidebands. A sideband of the modulated laser light is selectively detected and processed to produce an error signal which has a sign and magnitude that are functionally dependent on the direction and magnitude of the difference of the laser wavelength from the desired value.
The method taught by Mead overcomes the aforementioned difficulty of not having an error signal indicating the direction of laser wavelength error. However, Mead's teachings can only be applied for lasers operating in continuous wave (CW) or quasi-CW mode. Very brief light pulses cannot be modulated at a microwave frequency. Since very brief light pulses are essential for undersea communication, a method for efficiently stabilizing the wavelength of pulsed laser sources is needed.
Therefore, according to this inventive concept a need has been discovered for a way to stabilize laser output wavelength which provides an error signal indicative of direction and magnitude of wavelength error, and which can be utilized with both pulsed and CW lasers. Applications include undersea communication, photochemical isotope separation, and many other laser applications requiring that the laser light wavelength be precisely controlled.